Envelope Versus Fenestration: Backstops Drive Improved Thermal Performance

By Helen Sanders

Backstops are becoming more popular in U.S. building energy codes. Generally related to the envelope, their purpose is to set a minimum performance level for its thermal performance.

Envelope Backstops

When following the building performance compliance path, envelope-specific backstops limit how envelope thermal performance can be degraded and compensated with higher-performing internal systems, such as lighting and HVAC.

The area-weighted U-factor of the proposed building envelope typically can’t be worse than a specified percentage greater than the prescriptive envelope. The specific percentage varies from code to code and currently ranges from zero (e.g., in Massachusetts) to 20% (e.g., in Washington State). Some backstops, such as ASHRAE 90.1’s recent addendum, also allow credit for orientation and shading for greater flexibility.

By limiting how bad envelopes can be, envelope backstops reduce:
• Condensation risk and mold-related poor air quality;
• Occupant discomfort;
• The risk of death in extreme hot and cold weather due to the inability to maintain life-sustaining conditions during power outages; and
• Peak power loads, supporting grid resilience.

Some argue that backstops do not save energy since the same energy target needs to be met with or without it. However, when thermal bridging is ignored and air-leakage testing is limited, envelope performance is already over-estimated. This contributes to the well-documented “building energy performance gap” between as-simulated and as-built.

The envelope is typically one of the longest-life parts of a building, and the consequences of poor fenestration choices live on much longer than those of lighting and HVAC systems. Ensuring the envelope isn’t further degraded seems sensible.

A downside of envelope backstops is that they pit fenestration against opaque walls. As a result, this can potentially constrain the window area if there are insufficient investments in fenestration performance.

Fenestration Backstops

The Façade Tectonics Institute proposed a new type of backstop for the International Energy Conservation Code 2024 and California’s Title 24—a fenestration backstop. This eliminates the problem of constraining the window area because the performance trade-off limit is tied only to the fenestration.

The area-weighted U-factor of each fenestration type (fixed, operable) in the proposed design would not be allowed to exceed 110% of the prescriptive requirement. For example, if the prescriptive U-factor for fixed fenestration is 0.38 BTU/°f.hr.ft², then the area-weighted average U-factor for fixed fenestration in the design could be no worse than 0.42 BTU/°f.hr.ft². Treating fixed and operable fenestration separately still encourages the use of operable windows.

A Win for Glaziers

By not cheapening fenestration at the expense of improved HVAC systems, fenestration backstops can increase the glazier’s share of the project budget, revenue, and profit. It also supports the glazier’s early involvement at the design table alongside the HVAC contractor—an often-heard desire. The fenestration backstop offers all the benefits of an envelope backstop without the window-to-wall ratio downside.

Helen Sanders is the general manager of Technoform North America Inc. based in Twinsburg, Ohio. Read her blog each month at usglassmag.com/insights.

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